A295399 Numbers k such that (8*10^k - 611)/9 is prime.

3, 4, 7, 9, 48, 52, 105, 153, 1783, 1999, 2787, 4276, 13693, 14067, 19143, 20431, 25933, 26206, 29062, 40462, 40791, 58738, 81709

Offset

1,1

Comments

For k > 1, numbers k such that k-2 occurrences of the digit 8 followed by the digits 21 is prime (see Example section).

a(24) > 2*10^5.

Links

Table of n, a(n) for n=1..23.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 8w21.

Example

3 is in this sequence because (8*10^3 - 611)/9 = 821 is prime.
Initial terms and associated primes:
a(1) = 3, 821;
a(2) = 4, 8821;
a(3) = 7, 8888821;
a(4) = 9, 888888821; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(8*10^# - 611)/9] &]

Prog

(PARI) isok(k) = isprime((8*10^k - 611)/9); \\ Michel Marcus, Nov 22 2017

Crossrefs

Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

Sequence in context: A218278 A101062 A359394 * A257507 A003669 A047342

Adjacent sequences: A295396 A295397 A295398 * A295400 A295401 A295402

Keyword

nonn,more,hard

Author

Robert Price, Nov 21 2017

Extensions

Incorrect a(1)=2 removed by Georg Fischer, Jun 26 2020

Status

approved